PHYTOPHTHORA CROWN, COLLAR, AND ROOT ROT OF APPLE AND CHERRY WSU TREE FRUIT IPM STRATEGIES Written by S. Tianna DuPont, Tree Fruit Extension Specialist, Agricultural and Natural Resources, WSU; Shashika Hewavitharana, Associate Professor, Cal Poly; and Mark Mazzola, Research Plant Pathologist, USDA-ARS FS322E
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PHYTOPHTHORA CROWN, COLLAR, AND ROOT ROT OF APPLE AND CHERRY
WSU TREE FRUIT IPM STRATEGIES
Written by S. Tianna DuPont, Tree Fruit Extension Specialist, Agricultural and Natural Resources, WSU; Shashika Hewavitharana, Associate Professor, Cal Poly; and Mark Mazzola, Research Plant Pathologist, USDA-ARS
FS322E
WSU EXTENSION | PHYTOPHTHORA CROWN, COLLAR, AND ROOT ROT OF APPLE AND CHERRY
FS322E | PAGE 2 | PUBS.WSU.EDU
Phytophthora rots can girdle the scion (collar rot),
damage the rootstock just below the soil surface
(crown rot), and cause necrosis and death of fine
roots (root rot). While generally considered a larger
problem in rain-fed growing regions with heavy
soils, Phytophthora problems occur in Washington,
especially where irrigation water carries the
pathogen or where irrigation and overhead cooling
practices create wet soil conditions for extended
periods.
Multiple species of Phytophthora have been
implicated in crop damage with P. cactorum and P.
syringae two of the more significant species. P.
cambivora and P. citricola have also been isolated in
Washington from symptomatic trees (Yamak et al.
2002; Mazzola and Brown 2010).
Symptoms
Trees impacted by crown and root rot may appear to
have a slow general decline that from a distance can
be confused with winter injury, nutrient deficiency,
fire blight, or wet feet (where saturated soils reduce
the oxygen level to the point where roots die)
(Figures 1–4). Susceptible varieties may decline
more quickly.
When Phytophthora rots are suspected, inspect the
base of the tree and excavate soil from around the
trunk and root system to examine tissues for crown
and root rot symptoms. Upon peeling back the outer
bark of the trunk, the normally green cambium will
appear orange or brown. Roots may be a dark brown
or orange and may slough off from the primary root.
After cutting away the outer bark, a sharp line of
demarcation is commonly apparent between the
healthy and diseased (orange-brown) tissue.
In comparison, roots killed by “wet feet” or
drowning generally display an overall brown color
and may have a decaying or fermenting smell.
Crown rot (Figure 2) is used to describe the disease
when the pathogen affects the lower section of the
trunk at the soil line, whereas root rot primarily
impacts the tree root system. Collar rot may appear
as a dark brown, grey, or purplish depressed canker
on the bark of the lower part of the scion which can
be confused with fire blight (Figure 4). Trees may
have both root and crown rot symptoms.
Figure 1. Crown and root rot symptoms on apple.
Figure 2. Crown rot symptoms on apple. Note healthy tissue below the soil line.
WSU EXTENSION | PHYTOPHTHORA CROWN, COLLAR, AND ROOT ROT OF APPLE AND CHERRY
FS322E | PAGE 3 | PUBS.WSU.EDU
Figure 3. Cold injury on apple can be confused with crown rot.
Figure 4. Fire blight rootstock infection on apple can be confused with crown rot.
Causal Organism and Disease Cycle
Several species of the soil-borne oomycete
Phytophthora (fungal like organism) can cause root
and crown rots. Many of these species are common
in most orchard soils. Others can be introduced to an
orchard site in contaminated soil, irrigation water, or
planting stock. Phytophthora spp. persist in the soil
as either long strands of mycelium in infected plant
material or as thick-walled spores called oospores
(Wilcox 1992). These oospores are extremely
resilient and can remain viable in the soil for long
periods of time. When soils are wet, oospores
germinate forming thin threads of hyphae which can
infect plant roots.
When soils are saturated, another type of spore
called a zoospore develop within a structure called a
sporangia. Zoospores are swimming spores. They
can move through the soil water to find new roots to
attack. However, the soil must be flooded in order
for them to swim through the soil and as such
zoospores will only develop when the soil is
saturated. Spores can also move with water across
the surface of the soil in irrigation or run-off, thus
transmitting the pathogen over large parts of a block
quickly. The longer the soil stays wet the higher the
risk of infection.
Cultural Controls
Limit periods of soil saturation.
Free water allows pathogen zoospores to move
through the soil thus enabling disease transmission
over longer distances. Monitor soil moisture and
limit the amount of time a soil stays saturated.
Irrigate to field capacity when soil moisture and tree
monitoring indicate the need, and apply appropriate
volumes without flooding soils. When over-head
cooling is not properly cycled, soils may remain wet
or saturated during the cooling season of July and
August greatly increasing the risk of infections. Set
your cooling system so that cycles appropriately cool
fruit through evaporation. At this level little cooling
water should reach the ground.
WSU EXTENSION | PHYTOPHTHORA CROWN, COLLAR, AND ROOT ROT OF APPLE AND CHERRY
FS322E | PAGE 4 | PUBS.WSU.EDU
Choose well drained soils.
Whenever possible plant fruit trees in well drained
soils.
Maintain and improve soil structure.
Soils with high organic matter, aggregate stability,
water infiltration, and low compaction are less likely
to stay at overly wet levels for long periods. Adding
organic matter in the form of compost, trimmings,
grass clippings, and other material will build organic
matter over time.
Monitor irrigation water.
Irrigation water can be a source for Phytophthora
infections. Fruit infections (sprinkler rot) of pear and
apple was widespread in the Wenatchee River Valley
of Washington when overhead sprinklers were
common (1992–1999). Dr. Grove and colleagues
found 749 isolates (at least nine distinct taxa) of
Phytophthora spp. in irrigation canals in eastern
Washington (Yamak et al. 2002). Monitoring of
Phytophthora in irrigation water is generally done by
suspending fruit in the canal (or water from the
irrigation system) and checking for visible fruit rots.
Choose resistant rootstock.
Among dwarfing-apple rootstocks, Geneva series
rootstocks are most resistant. M-9, M-2, and M-4 are
moderately susceptible. M-7 and MM-111 are
moderately susceptible; M-26 and MM-106 are
susceptible; and MM-104 is highly susceptible
(Table 1). Among stone fruits, plums are relatively
resistant, whereas the remainder are susceptible to
very susceptible. Mahaleb is the most susceptible
cherry rootstock, whereas Mazzard, Morello, and
Colt are somewhat more resistant and would be
recommended on heavier soils.
Biological and Biopesticide Controls
Maintain large, active beneficial populations.
Many soil bacteria and fungi can be antagonistic to
root rot fungi. For example, a fungus called
Trichoderma has been seen to reduce soil-borne
disease infections. In general, additions of
Table 1. Relative resistance of apple rootstocks to Phytophthora root and crown rot.